Research on Hard-Rock Mining in Mountainous Terrain

The Toxics Program research efforts on hard-rock mining in mountainous terrain
has had a change in emphasis in the last several years. The major change has
been a concerted effort to work in the pilot watersheds of the USGS Abandoned
Mine Lands (AML) Initiative -- the Animas River in Colorado and the Boulder
River in Montana. These are mountainous watersheds, with pool-and-riffle streams
that receive natural and mine-drainage inflows of surface and ground water.
Our role in the AML Initiative has been to adapt and transfer technology developed
through field experimentation in St. Kevin Gulch, the Upper Arkansas River watershed,
Colorado. This entails adaptation of research methods to the watershed-scale
problems faced by Federal land managers in practical field situations.

CHANGING SCALE TO THE WATERSHED

Tracer-injection and synoptic sampling studies in St. Kevin Gulch were on the
scale of 2 kilometers (km) or less and at streamflows of less than 1 cubic foot
per second (cfs). In this setting, changes in streamflow along the study reach
generally were less than an order of magnitude. For the AML Initiative, we have
had to adapt methods to a scale of up to 12 km, and streamflow up to 200 cfs,
resulting in more than an order of magnitude change in streamflow. This has
not meant simply doing a "big" St. Kevin Gulch test by mixing more tracer and
injecting it at a greater rate. Transitions from small to large order streams
within the basins have required careful evaluation to break the study reach
into workable segments. This keeps the tracer concentrations within a reasonable
analytical range for each reach, and enables effective interpretation of the
tracer-test results.

New sites and different scales have required new thinking and new logistics;
each stream has provided new insights. Despite the change in scale, we have
found the need to maintain spatially intensive sampling to adequately describe
mass balance for inflows to streams. This enables unique identification of various
contamination sources and linkage to distinct stream segments within a watershed,
information essential for remediation design and monitoring.

UNDERSTANDING INFLOWS

Characterization of the water chemistry of stream inflows has always been an
important goal of synoptic sampling. We are able to identify net water-quality
changes for stream reaches and to quantify the average concentrations of metals
in inflows to these reaches. Characterization of inflows has enabled linkage
of the impacts observed in the stream with specific sources of mine drainage.
Thus, the goal to understand inflows becomes a goal to understand the pathway
of contaminants from a source to the stream. The pathway depends on catchment
hydrology as it is influenced by geologic structure. We are expanding our inflow
sampling to observe both surface- and ground-water pathways, to make the link
between contaminant sources and the streams.

SIMULATING REMEDIATION OPTIONS

Reactive solute-transport models (OTEQ) have been applied to explain the changes
in water chemistry downstream, revealing an ability to design and evaluate remediation
options. The removal of a mine drainage source is not simply removal of a source
term in a contaminant mass-balance exercise. Contaminants are affected by complex
reactive processes that occur within the stream. Removal of a metal load can
change pH conditions, and in turn, change the overall reaction processes within
the stream. Thus, simulation of a remediation option might consider its effect
on overall stream chemistry and natural attenuation of metals occurring in the
stream. As many options are evaluated with models, we will gain needed understanding
of how the many efforts to characterize the watershed fit together to help guide
remediation and monitoring efforts.

The papers of this session and two in the session on the USGS Abandoned Mine
Lands Initiative reflect progress on these issues. For additional information
contact: Briant A. Kimball, USGS, Salt Lake City, Utah, (email: )